Soft Recovery Process of Mechanically Degraded Flexible a-IGZO TFTs With Various Rolling Stresses and Defect Simulation Using TCAD Simulation

Abstract

We examine the effects of repetitive rolling stress on thin-film transistors (TFTs) using various rolling radii, and evaluate the subsequent thermal treatment at various temperatures. After applying repetitive rolling stress, the electrical characteristics of the TFTs changes. In particular, change of subthreshold swing (SS) is significant than that of threshold voltage (Vth), and saturation mobility ( $\mu _{\text {sat}}$ ). We simulated the electrical performance of TFTs using technology computer-aided design (TCAD) to quantitatively ascertain the change to donor-like and acceptor-like defects in the active region. Both Gaussian donor-like defects (NGD) and acceptor-like defects (NGA) increased under mechanical strain due to the formation of oxygen-related defects, oxygen vacancy (Vo), and oxygen interstitials (Oi). In addition, negative bias illumination stress (NBIS) reliability results show that degraded TFTs at higher rolling stress yield more variation of threshold voltage ( $\Delta \text{V}_{\text {th}}$ ) and $\Delta $ SS due to increase in interface trap site, and defect sites. The rolling stress generates more oxygen-related defects, Vo and Oi, which act as TFT degradation factors. Finally, we investigate the possible recovery mechanism of mechanically degraded TFTs through thermal treatment. Thermal treatment at 150 °C and 250 °C cannot recover the electrical performance of mechanically stressed TFTs and NBIS reliability to before rolling state. Therefore, we used TCAD simulation to speculate the origin of incomplete recovery. We thought that this incomplete recovery originated from oxygen dimers (O-O) formed from Oi under mechanical stress.